00:00
What are the neurological changes that occur? These mainly have to do with the brain size and
the amount of myelination that occurs on axons. Let's look at a couple of the different years
and go through some of the responses. So at 40 weeks we're about at 400 g. It's pretty
rapid in terms of a gain to about 1000 g within the first year. It then tapers off some
that the rate arise and when you get into adolescence maybe you're somewhere around
1250 to 1400. Myelination is going to be occurring throughout childhood. It depends on the
various structure that we're dealing with how quickly it takes to myelinate but this is one
reason why coordination develops and some other brain functions work a little bit better
as one is maturing. Other neurological changes that happen are being able to regulate one's
own body temperature. In fact, there is a circadian rhythm associated with temperature
regulation and early on within the first few months you have a lower nightly temperature
but when 3-6 months that magnitude starts to change and finally it takes all the way to about
year 2 to maybe even 5 before the adult normal circadian rhythm variation is able to be
observed. Infants also have an impaired ability to shiver. They don't have good muscle
coordination as of yet so therefore they can't utilize shivering in thermogenesis. What they do
use, however, is something called brown adipose tissue. So there are brown adipose tissue sites
especially around some of the large organs that go through a futile cycle in which in response
to norepinephrine there is increased heat production. The final thing that's different for
thermoregulation as a child develops is their surface area to mass ratio. The more surface area
they have, the greater ability they have to lose heat. The less surface area they have,
the more they retain heat. So you have to factor that into the person's growth patterns.
02:14
So when they have long limbs, they are able to radiate heat better than when they have short limbs.
02:21
There are also sleep-related changes. A neonate sleeps quite a bit 16-17 hours per day.
02:28
In fact, the sleep cycles are fairly short, only 45-60 minutes. They spend a lot of time in
REM sleep. If we contrast that to the infant, they start to sleep for longer periods at a time
maybe 8-10 hours and by year 1 they're dropping the amount of REM sleep that they need
to about 40%. A child will sleep about 10-12 hours a day so still a little bit longer than an adult,
but now they are starting to gain a normal sleep pattern in which they're going to go to the
various stages of sleep every 90-110 minutes. There are some blood-related changes
associated with development. A neonate has a very high hematocrit. In fact, they look like
they have polycythemia. They have a hematocrit of about 50%. Hemoglobin levels are also high
per deciliter. In early and late childhood hemoglobin drops so in this case it is 30 to maybe
upwards to 38 at late childhood and hemoglobin levels are still fairly low if we compare that
to the normal adult male or woman. Here, hematocrits are somewhere in between about
40-54% for male and 37-47% for females. The cardiovascular system changes quite a bit
as one develops. Neonates, when they first start out, have a very large right ventricle. Because
this was hypertrophied while the fetus was in the womb and had to overcome very high
pulmonary vascular resistances but as soon as birth happens now you are having a low resistance
and so the right ventricle does not have to work as hard to go through the pulmonary
vasculature. The infant's side difference starts to equate especially because systemic vascular
resistance becomes so much higher but this also involves a large mass change in the heart
maybe about doubling by year 1. By late childhood there's probably about a 6-fold increase
in the amount of heart mass or heart tissue. If we look at some of the standard heart rates
and blood pressure responses, if you look at a preemie you have a fairly high heart rate
120-170 and a pretty low systolic blood pressure. You can see this will start to drop in terms of
the amount of heart rate as you have an infant to 1 year old and this keeps dropping as one
goes from 4, to 6, to 8, to 10 until you reach adult levels around 12 years of age. The minimum
systolic blood pressure continues to climb while heart rate is dropping. Let's go through an
example of the change that could happen in cardiac output if we compared an infant to an
adult. So in an infant you have a very small stroke volume. That's the volume of blood
ejected per beat versus a heart rate which is how many times the heart beats per minute
so you have an overall cardiac output of around 1.3 L. If we contrast that to an adult who has
a stroke volume of around 70 mL/beat and a heart rate about 72, you get a cardiac output
around 5 L. So there are quite a bit of cardiac output difference between infants and adults.
06:16
The other thing that's very different about this response is the ability to change stroke volume.
06:23
During the more maximal stimulation, stroke volume may only go up about 3 mL/beat.
06:29
Therefore, even if you had a higher heart rate you're cardiac output may only increase to
2 L. If we contrast that to an adult who has the ability to really change stroke volume
by 50 mL/beat, you get large changes all the way up to 19.2 L/min. Highlighting the differences here
are stroke volume in nature, the ability to increase stroke volume and the absolute stroke
volume you start with. There are also changes that occur with the respiratory system.
07:06
There are higher respiratory rates, which is your breathing frequency in childhood. There are
lower lung volumes and capacities. There are also lower abilities to diffuse gases across
the lung but these seem to be fairly proportional to size and why that is important is then it
means you don't have a diffusional lung capacity test or problem but rather it is related only
to the size of the lungs. By about year 3 is about the first time you're able to really complete a
lung function test. Prior to this, it's very difficult to get someone to do the type of breathing
procedure necessary to be able to undergo a respiratory test such as a pulmonary function test.
07:57
So what's a normal ventilation rate? If you look at a preemie or looking at somewhere between
40-60 and this will gradually drop as you age until you get to about a normal rate at about
age 12 around 12-20 breaths per minute. Other respiratory changes that happen are there are
smaller airways in younger children. This means that it's easier to obstruct their airways
and that obstruction could be from a foreign object such as something that was aspirated
or could be clogged up from some of their own secretions. Another potential problem is that
neonates have non-fully calcified ribs and this makes the compliance of the lungs a little bit
different so they have to enact different breathing patterns because of this high compliance
and once calcification of the ribs happen, they will be able to return to more of a normal
respiratory rate. There are also renal changes that occur from neonates to infants to children.
09:17
The biggest thing about a neonate just after birth you have a very low glomerular filtration rate
and your ability to concentrate urine is really pretty poor. Infants get a little bit better in
being able to have more glomerular filtration rate as compared to their size but they still
can't concentrate their urine like an adult. In fact, it takes a while for that ability to
concentrate urine comes into play. Children also have a reduced ability to excrete potassium
to reabsorb bicarbonate and then to totally buffer hydrogen ions. To buffer hydrogen ions
that's working with non-volatile acids. So let's take this idea of inability to fully concentrate
the urine and let's show a couple examples of that. So in a normal adult, you have a nephron
loop. The nephron loop is an index of how much concentration abilities you have so in adult
you might go from an isoosmotic portion of the nephron about 300 milliosmoles all the way
down to the hairpin loop which is 1200 milliosmoles, so we are able to concentrate urine by
about 900 milliosmoles. If we compare the neonate, they're only able to go from 300 to maybe
450 less of an ability to concentrate their urine. What does that mean? The fluids that they
take in they're more likely to urinate out without holding on to that fluid. They don't reabsorb
their sodium and water to the same level so it's easier for them to become dehydrated.
11:12
Infants are a little bit better, they can drop their osmolality to maybe somewhere around 900
but you still don't have the full adult ability to fully concentrate the urine so again still
infants have that ability to become dehydrated as well because they cannot hold on to their
water in times of which they are water challenged. This means that we really have to be
careful of water turnover rates in neonates and infants. They have a lower ability to
concentrate urine. The other item that we need to think about for homeostasis for acid-base
balance is that since usually children have a high metabolic rate they're using ketone bodies
to a greater extent and their kidney has a less of an ability to deal with non-volatile acids,
they are prime for things like metabolic acidosis. Because they don't have that ability to
buffer those hydrogen ions yet, you have to be very careful with acid-base balances in young children.